Tableau Online does not support SSO via trusted tickets or AD, so while you could point to an embedded viz with a SharePoint Page Viewer web part, the user is going to need to login within the web part itself.
Hope this helps,
Thanks for your help Russell
This is a really old post, but I stumbled upon it today and wanted to chime in just in case anyone comes across the post.
Since the introduction of Site Specific SAML in Tableau, a seamless single-sign-on experience is possible when embedding Online vizzes.